The present invention relates to a method for measuring blood pressure, and to a blood-pressure measuring device for carrying out the method.
Blood pressure is normally measured with the aid of a blood-pressure cuff or sleeve which is applied around the upper arm of the patient (or sometimes around the patient's thigh).
The prevailing method of measuring blood pressure involves the use of a universal cuff, which is used for all patients. Research within this field, however, has shown that serious measurement errors can occur when measuring with the aid of an universal cuff, since systematic errors are added to the true blood pressure value.
Due to the presence of soft tissue between the blood vessel and the bone in the centre of the arm (or the thigh) around which the cuff is placed, the use of one and the same cuff with arms of mutually different sizes or thicknesses does not give a clear reading of the true blood pressure. In the case of a thin arm, the pressure read-off will be relatively low, whereas in the case of a thick arm, the pressure read-off will be relatively high, even if that in reality the persons concerned have mutually the same blood pressure.
U.S. Pat. No. 3,812,844 teaches a blood pressure cuff incorporating automatic compensation of the measured values in dependence on variations in the diameter of the measurement object. This is effected by dividing the blood pressure cuff into a plurality of longitudinally extending sections, each of which is associated with a respective indicating range on the measuring device. This requires the use of a special blood pressure cuff in order to obtain a corrected measurment value, this value only being corrected with respect to the circumference of arms and not with respect to the remaining parameters aforementioned.
It is also found that the pulse frequency of a patient at the time of measuring blood pressure will also influence the blood pressure value measured with the universal cuff. At high pulse frequencies, the universal cuff tends to indicate an excessively high pressure value. This may depend, for instance, on the extent to which the heart is filled prior to blood being pumped therefrom. At high pulse rates, the heart is not completely filled, and consequently the heart beats will not be as powerful as heart beats at low pulse frequencies.
When measuring the blood pressure of elderly people with the aid of an universal cuff in a conventional manner, the true blood pressure value is exaggerated or overestimated, i.e. the pressure value read-off is somewhat higher than the true blood pressure value. This is probably due to the fact that the veins and arteries become stiffer with age.
It is also found that when measuring blood pressure with an universal cuff, the blood pressure of women is exaggerated overestimated in comparison with blood pressure of men, i.e. if a man and a woman have equal blood pressures in reality, the blood pressure reading in the case of the woman will be higher than that for the man.
The pressure level of the patient at the time of measuring blood pressure will also influence the blood pressure value measured by the universal cuff, wherein a low blood pressure results in depreciation of the true blood pressure, whereas a high pressure level results in exaggeration of the true blood pressure.
The systematic errors contingent on variations in the aforesaid parameters: age, sex, arm circumference, pulse frequency and pressure level, cause the blood-pressure values obtained with the universal cuff to incorporate in reality errors in the order of 5-10 mm Hg with respect to both the systolic and diastolic pressure.
The limit for treatment of a patient for, e.g., hypertony, i.e. increased blood pressure, may depend on whether the patient has a lower pressure, i.e. a diastolic pressure, of 95 or 100. Thus, the systematic errors occurring when using the universal cuff are decisive in many instances in deciding whether a patient is sick or well and whether the patient requires treatment or not.
The present invention attempts to solve the aforesaid problems, by introducing corrections for the aforesaid systematic errors occurring when measuring blood pressure with an universal or unitary cuff, thereby to eliminate the error margin to the greatest possible extent.